The present invention is directed, in general, to battery backup systems and, more specifically, to a low voltage cutoff circuit with short circuit detection capability, a method of operating the same and a battery backup system incorporating the circuit or method.
Commonly, many existing battery backups for telecommunications systems do not possess circuitry for protecting the system from low voltage or undervoltage conditions. In such systems, a main power source goes off-line and a battery backup supplies power necessary for system operation. While operating on backup power, the overall voltage supplied by the batteries will slowly drop. Once the backup voltage drops below a known level, irreversible battery damage occurs.
To combat this problem, various new battery backup systems contain sensing equipment that monitors a load voltage. When operating on backup power, the sensing equipment monitors the load voltage via sense leads in a feedback-based configuration. Once this load voltage drops to an unacceptable level, the entire system, including the battery backup is shut down.
Problems with these systems stem from an economic and service origin. To implement such battery backups, users would be required to replace existing backups with the new systems. This option is, from a cost standpoint, completely unacceptable. Further, in order to replace the backups, the associated telecommunications equipment would most likely suffer repeated interruptions.
Another option to overcome the problems associated with low voltage or undervoltage conditions includes placing modules containing sensing equipment that monitors battery voltages from the load. Once the supplied battery voltage drops to an unacceptable level, the load or loads are disconnected.
A reoccurring problem encountered in these modules involves the device used to interrupt the telecommunications system. The device used to interrupt the battery voltage to the load is typically a field effect transistor (FET). A FET is commonly used since it is very small and more reliable compared to electromechanical counterparts. The problem with low voltage cutoff traditionally lies in short circuit protection upon power-up. If a short circuit were applied to the load side of the module after power up, the fuse would blow and the system would shut down. If the short circuit were to remain on the load or if there was an unknown short circuit on the load prior to power-up, the FET would be destroyed when the module attempts to apply the battery voltage to the load. This is commonly termed a xe2x80x9csilent failure.xe2x80x9d The FET fails in a shorted condition, battery voltage continues to be delivered to the load, almost certainly damaging circuit elements throughout the module. The user is not aware of the failure until the battery plant voltage becomes too low and the system as a whole fails.
Accordingly, what is needed in the art is a way to protect against such xe2x80x9csilent failures.xe2x80x9d
To address the above-discussed deficiencies of the prior art, the present invention provides a low voltage cutoff circuit, a method of operating the same and a battery backup system incorporating the low voltage cutoff circuit or the method. In one embodiment, the low voltage cutoff circuit includes: (1) a low voltage monitor coupled between an input and an output of the low voltage cutoff circuit, (2) a cutoff switch, coupled between the input and the output and controlled by the low voltage monitor, that closes to couple the input to the output, the cutoff switch subject to failing closed when a voltage of the input is below a threshold and a load couplable to the output contains a short circuit and (3) a short circuit protection circuit, coupled to the low voltage monitor, that senses when the load contains the short circuit and directs the low voltage monitor to prevent the cutoff switch from closing.
The present invention therefore introduces the broad concept of protecting a cutoff switch as against a potentially harmful overcurrent condition that a short circuit in a load may cause. The overcurrent condition, which could cause the cutoff switch to fail closed, is avoided by preemptively preventing the cutoff switch from closing when a short circuit in the load is detected.
In one embodiment of the present invention, the cutoff switch is a field-effect transistor (FET). Those skilled in the art are aware of the tendency of FETs to fail closed under overcurrent conditions. However, those skilled in the art should understand that the present invention is fully employable with other types of cutoff switches, such as electromechanical relays, which may also be subject to failing closed.
In one embodiment of the present invention, the short circuit protection circuit comprises a short circuit sensor that is noninvasive with respect to the load. A xe2x80x9cnoninvasivexe2x80x9d sensor is defined for purposes of the present invention as a sensor that does not require a resistor or other device to be series-coupled with the load. A current transformer is noninvasive, but an in-line diode is not. Of course, the broad scope of the present invention is not limited to noninvasive sensors.
In one embodiment of the present invention, the low voltage monitor opens the cutoff switch only when the voltage of the input is less than the threshold. In an embodiment to be illustrated and described, a fuse is located between the source and the low voltage monitor. When the voltage of the input is at or above the threshold, it is desired to blow the fuse rather than open the cutoff switch. Of course, the cutoff switch can be opened after the fuse blows or instead of blowing the fuse.
In one embodiment of the present invention, the low voltage cutoff circuit further includes a power filter coupled between the input and the cutoff switch. Those skilled in the art are familiar with the structure and function of power filters.
In one embodiment of the present invention, the low voltage cutoff circuit nominally carries electric power at about 48 volts and the threshold is less than about 45 volts. The 48 volt nominal operating voltage is typical for operating telecommunications equipment. A 43 volt threshold is advantageous in the embodiment to be illustrated and described.
In one embodiment of the present invention, a source of electrical power is coupled to the input. In the embodiment to be illustrated, the source is at least one battery.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.